Evolutionary Medicine Notes

Evolutionary Medicine

  • Definition

    • Evolutionary medicine is the integrated study of evolution and medicine aimed at improving scientific understanding of the reasons for disease and actions that can be taken to improve health.

Evolution and Pathogens

  • Pathogen Evolution

    • Pathogens evolve faster than their hosts.

    • This rapid evolution can lead to the emergence of new diseases.

    • Natural selection often lags behind environmental change.

    • Trade-offs in biological processes make it difficult for natural selection to resolve certain health issues.

    • Evolutionary history constrains potential changes by natural selection.

    • Certain traits can enhance reproductive fitness but may increase vulnerability to disease.

    • Some diseases may actually be adaptive responses, rather than purely detrimental.

Medicine and the Limits of Natural Selection

  • Natural Selection and Health

    • Natural selection can eliminate deleterious mutations and spread beneficial ones over time.

    • It has shaped the evolution of complex adaptations.

    • Humans are not perfect; evolutionary processes have left us vulnerable to various ailments.

  • Understanding Maladaptations

    • Maladaptations, such as issues with the human eye, inform us about diseases and our vulnerabilities to them.

    • Vulnerabilities can be categorized into six evolutionary explanations.

    • These explanations will be utilized to better understand diseases like malaria, cancer, and obesity.

Pathogen Dynamics

  • Pathogen Population Dynamics

    • Pathogen populations can respond swiftly to selection pressures, leading to rapid diversification.

    • Example of HIV:

    • HIV can produce approximately 1 billion descendants per day.

    • Lacks the genetic repair machinery to fix replication errors, resulting in increased genetic diversity.

  • Evolving Pathogens

    • Pathogens evolve to escape immune responses, thereby causing continued disease.

    • In a host environment, mutations that improve resource utilization are favored by natural selection.

    • An example: A virus can acquire mutations that enhance its ability to hijack infected cells, increasing its replication success.

    • Immune responses provide additional selective pressure favoring mutations for evasion.

Pathogens and Human Hosts

  • Examples of Adaptation

    • Pseudomonas aeruginosa

    • A species that can transition from a harmless state to becoming pathogenic, particularly in individuals with cystic fibrosis.

    • Cystic fibrosis leads to an environment which favors survival and proliferation of Pseudomonas due to excess mucus and inflammation.

  • Adaptive Evolution in Pathogens

    • As Pseudomonas enters the lungs, it must adapt to the low-oxygen environment, significantly altering its virulence profile.

Evolution of Virulence

  • Understanding Virulence

    • Virulence is defined as the level of harm a pathogen causes its host while using the host's resources.

    • Variability exists among species.

    • Example: The flu versus the common cold.

    • Virulence can evolve due to two opposing selection pressures:

    • Selection for rapid replication (within-host) can lead to increased virulence.

    • Selection for transmission (between hosts) may favor reduced virulence.

  • Case Study: Salmonella typhimurium

    • Through serial passage in mice, mild strains of Salmonella evolved to highly virulent strains that caused mortality in infected hosts.

Implications for Public Health

  • Understanding evolutionary dynamics in pathogens guides intervention strategies.

  • Improving hygiene, drinking water, and vaccination rates can influence pathogen selection processes, potentially leading to lower virulence strains.

Antibiotic Resistance

  • Historical Context

    • Mid-1900s: Discovery of antibiotics revolutionized treatment of infectious diseases.

    • Initial optimism suggested infectious diseases would soon be eradicated.

Emergence of New Pathogens

  • SARS Case Study

    • Emergence of SARS in 2002 traced back to a new virus observed in a Chinese farmer.

    • Global spread stemming from infected individuals traveling.

    • Investigations revealed connections to masked palm civets and subsequently to bats as the likely original host.

  • Evolutionary Patterns

    • Pathogens may have lengthy co-evolutionary histories with human ancestors or may shift hosts recently, emphasizing the constantly changing nature of disease causation.

Sickle Cell Anemia: An Evolutionary Perspective

  • Genetic Basis

    • The HbS allele of the β-globin gene is responsible for sickle cell anemia.

  • Disease Mechanism

    • Two copies of the HbS gene lead to sickle-shaped cells that can obstruct blood flow, causing severe health issues.

    • Carriers (HbA/HbS) experience a milder form, exhibiting some resistance to malaria.

  • Natural Selection and Malaria

    • HbS carriers show a selective advantage in malaria-endemic areas, maintaining gene frequency despite potential health risks associated with full sickle cell disease.

    • Demonstrates an evolutionary trade-off: carrying the sickle cell trait may confer malaria resistance while increasing the risk for sickle cell anemia.

Cancer from an Evolutionary Perspective

  • Causes of Cancer

    • Cancer involves mutations to oncogenes and tumor-suppressor genes, compounded by environmental exposure.

    • Development and susceptibility depend on inherited genetic factors and environmental agents.

  • Natural Selection in Tumors

    • Tumors evolve under selective pressures, with mutations allowing for resistance to treatment becoming prevalent over time.

    • Chemotherapy acts as a selection pressure, sparking the rapid evolution of resistant cell lineages.

Elephants and Cancer Resistance

  • Elephants as a Case Study

    • Elephants have multiple p53 gene copies (20) compared to humans (1).

    • The LIF6 gene in elephants contributes to cancer resistance by targeting and destroying mitochondria in damaged cells.

  • Evolutionary Implications

    • Insights from the study of elephants may inform future cancer treatments.

Feline HIV

  • Additional insights into the evolving resistance to diseases and comparisons in pathogenic dynamics across species.